Reciprocating valves in internal combustion engines are typically actuated by a rotating cam operating a push rod, the push rod pressing a rocker arm thereby to depress the valve or alternatively in an overhead camshaft system a rotating camshaft over the engine block engages an intermediate portion of a rocker arm pivoted at one end, the other end depressing the valve. Thus the shape of the cam lobe serves to determine the duration and rate of opening and closing of the valve and the valve stroke length.
A number of systems have been previously proposed, for instance in Australian Patent Application No. 82878/82, to provide a second rotating cam operating on a rocker arm which second cam can be actuated to provide further control over operation of an engine valve. Such systems, however, are quite limited in their application as only a small number of control parameters can be altered to affect operation of the valve. This limitation severely restricts the ability to make adjustments to the system when it is desired to optimise engine performance under different operating conditions.